std::getline throwing when it hits eof

Question

std::getline throws exception when it gets an eof. this is how I am doing.

std::ifstream stream;
stream.exceptions(std::ifstream::failbit|std::ifstream::badbit);
try{
  stream.open(_file.c_str(), std::ios_base::in);
}catch(std::ifstream::failure e){
  std::cout << "Failed to open file " << _file.c_str() << " for reading" << std::endl;
}
while(!stream.eof()){
  std::string buffer = "";
  std::getline(stream, buffer);
  //process buffer
  //I do also need to maintain state while parsing
}

In the above code getline is throwing exception as it gets eof How to handle this situation ?

EDIT

std::string buffer = "";
while(std::getline(stream, buffer)){
    //also causes getline to hit eof and throw
}

Sultry

Side note: the structure of the loop is incorrect as you need to check eof() immediately after the std::getline(). – Behaviorism

You are asking for an exception. After hitting eof, the stream is in a bad state. – Gummous

Why is iostream::eof inside a loop condition considered wrong? – Intisar

'How to handle this situation'. That depends on what you want to do, you haven't explained yourself very well. Do you want to not have an exception? Do you want to catch the exception, and if you want to catch it what do you want to happen next? Please explain what you are trying to do. – Rendarender

Answer 1

You activate the exception handling of your stream at the very beginning of your code:

stream.exceptions(std::ifstream::failbit|std::ifstream::badbit);

Now if the extraction of formatted data such as floating-point values, integers or strings will fail, it will set the failbit:

eofbit    indicates that an input operation reached the end of an 
          input sequence;
failbit   indicates that an input operation failed to read the expected 
          characters, or that an output operation failed to generate the 
          desired characters.

While getline(stream,buffer) will indeed set the eofbit if it reaches the end of a file, it will also set the failbit, since the desired characters (a line) couldn't be extracted.

Either wrap another try-catch-block around your loop or disable the failbit exception.

Example:

#include <iostream>
#include <fstream>

int main(){
  std::ifstream stream("so.cc");
  stream.exceptions(std::ifstream::failbit|std::ifstream::badbit);
  std::string str;

  try{
    while(std::getline(stream, str));
  }catch(std::ifstream::failure e){
    std::cerr << "Exception happened: " << e.what() << "\n"
      << "Error bits are: "
      << "\nfailbit: " << stream.fail() 
      << "\neofbit: " << stream.eof()
      << "\nbadbit: " << stream.bad() << std::endl;    
  }
  return 0;
}

Result:

Exception happened: basic_ios::clear
Error bits are:
failbit: 1
eofbit: 1
badbit: 0

Note that both eofbit and failbit are set.

See also:

• std::ios_base::iostate
• Table 124 in § 27.5.3.1.5 (see above)

Calcaneus

So should I place a silent catch block around getline ? – Sultry

If you use getline(std::istream,std::string), then the only reason for it to fail to extract the string will be an EOF. I suggest you to actually disable throwing an exception on std::ifstream::failbit. Of course you can use a silent catch block. There are many ways to handle such things, choose the one that suits you best. – Calcaneus

I want the getline read the stream into buffer as long as it doesn't fetch eof. once it fails due to eof` I want the control to come below getline because I've states of previous lines kept previous iterations. – Sultry

Answer 2

As you said, calling getline when the stream is currently positioned on the blank line at the end of the file will set both eofbit (because it reaches the end of file) and failbit (because nothing could be extracted, not even the line delimiter).

So, if you have set the stream to throw exceptions when failbit is set, you get an exception thrown when reading that blank line.

To prevent that, you can peek for the next character on the stream before calling getline:

std::string buffer = "";
while(stream.peek() != EOF && std::getline(stream, buffer)){
    //do something with the buffer
}

Backer

Did not have luck with this approach; I still get an exception after the peek. – Jit